1. Field of the Invention
The present invention relates to an organic electrolytic solution and a lithium battery employing the same, in particular, to an organic electrolytic solution and a lithium battery with enhanced reliability.
2. Discussion of the Related Art
A lot of research regarding batteries as a driving energy source has been conducted to minimize battery weight for, and meet sophisticated technology requirements of, potable electronic devices such as video cameras, cellular phones and laptop computers. Particularly, rechargeable lithium batteries have three times the energy density per unit weight as conventional lead storage batteries, nickel-cadmium batteries, nickel-hydro batteries and nickel-zinc batteries, and they can be recharged in a short time.
A lithium battery cathode is typically composed of an active material including transition metal compounds such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1-xO2, Ni1-x-yCoxMnyO2, or oxides containing the transition metal compounds and lithium. A lithium battery anode is typically composed of an active material including lithium metal, a lithium metal alloy or a carbonaceous material, and a graphite material. Electrolytes are categorized as liquid or solid electrolytes, according to electrolytic type. Liquid electrolytes raise many safety problems including the potential danger of fires due to the outflow and destruction of batteries from evaporation. Hence, many researchers have suggested using solid electrolytes instead.
Many studies have focused on solid electrolytes, and on solid polymer electrolytes particularly, because solid polymer electrolytes are unlikely to leak electrolytic solution, and they are easy to process. Solid polymer electrolytes are further categorized into full solid types and gel types, where the full solid types do not contain an organic electrolytic solution, while the gel types do.
Generally, conventional aqueous electrolytic solutions are not suitable for lithium batteries mainly because they may react violently with lithium, which is used as an anode. Thus, an organic electrolytic solution in which a lithium salt is dissolved is used instead. The organic solvent may have high ionic conductivity, a high dielectric constant and low viscosity. But it is very difficult to obtain a single organic solvent having all three of these characteristics. As a result, a mixed solvent composed of an organic solvent having a high dielectric constant and an organic solvent having a low dielectric constant, or a mixed solvent composed of an organic solvent having a high dielectric constant and an organic solvent having low viscosity, is used as is an organic solvent for lithium batteries.
U.S. Pat. Nos. 6,114,070 and 6,048,637 disclose a mixed solvent composed of a linear carbonate and a cyclic carbonate, such as a mixture of dimethyl carbonate or diethyl carbonate, and ethylene carbonate or propylene carbonate, to improve the organic solvent's ionic conductivity. In general, the mixed solvent can be used only at 120° C. or lower, because if the temperature rises above 120° C., a battery using the mixed solvent may swell due to the gas generated from its vaporization.
Alternatively, the utilization of 20% or greater of vinylene carbonate (VC) has been suggested as a main organic solvent of an organic electrolytic solution (U.S. Pat. Nos. 5,352,548, 5,712,059, and 5,714,281). When vinylene carbonate is used as the main solvent, however, charge/discharge characteristics may be degraded and high-rate characteristics may be decreased because vinylene carbonate has a lower dielectric constant than ethylene carbonate, propylene carbonate and γ-butyrolactone.
U.S. Pat. No. 5,626,981 discloses a battery in which a surface electrolyte interface (SEI) is formed on the surface of a cathode during initial charge/discharge due to VC in an electrolytic solution, and U.S. Pat. No. 6,291,107 discloses a battery in which a polymer film is formed on the surface of a carbonaceous anode material by a monomer capable of electrochemical anionic polymerization (anionic polymerization monomer) during the initial charging. In Japanese Patent Laid-open Publication No. 2001-223154, a vinyl-based compound such as vinyl acetate is introduced in the form of an additive.
However, improving the adhesion between the polymer film and the surface of the carbonaceous anode material is desired because the adhesion in conventional batteries may not be strong enough to appropriately inhibit side reactions, such as the decomposition of the electrolytic solution.